The present invention relates to a printing device in which characters or the like are printed on a printing sheet through a printing ribbon while a printing head is moved along the printing line.
Heretofore, in printing devices in which a two-way printing operation is carried out in association with the forward and backward movement of the printing head, when the printing head is moved forwardly, the printing ribbon is moved slightly in the direction opposite to the direction of movement of the printing head. The printing ribbon extended along the printing line is thus tightened. When the printing head is moved backwardly, the printing ribbon is moved in the direction opposite to the direction of movement of the printing head at a speed higher than the speed of movement of the head. An unused part of the printing ribbon is thus at the printing position at all times. This method eliminates low quality printing characters caused by slackened printing ribbon in the forward movement of the printing head and eliminates blurred characters caused by the used part of the printing ribbon being repeatedly struck in the backward movement of the printing head.
In the conventional printing device in which the printing ribbon is run as described above, the torque of the printing ribbon driving motor is smaller than that of the printing head driving motor. Therefore, when the printing head is moved backward in the same direction as the ribbon, the ribbon is moved faster than the printing head. As shown in FIG. 6, the rise time t2 required for the speed of the printing ribbon to reach a predetermined value is longer than the rise time t1 required for the speed of the printing head to reach a predetermined value. Accordingly, if when the speed of the printing head reaches the predetermined value, the printing operation is started, there is a period of time in which the speed of the printing ribbon is substantially equal to the speed of the printing head (as indicated by the shaded region in FIG. 6). The same part of the printing ribbon is therefore being repeatedly struck. As a result, the printed characters or the like are not clear, or of bad quality.